The nozzles of the lance heads for blowing oxygen have the task of providing the metal bath with the necessary amount of oxygen for refining cast-iron. The oxygen is to be conveyed to the bath with suitable quantities and manners; in fact, the metallurgical results of the conversion depend on e.g. the number and shape of the nozzles through which are formed the jets acting on the metal bath and interacting therewith. It is therefore necessary a careful fluid-dynamic study on the shape of the nozzles generating said oxygen jets.